Electromagnetic interference (EMI) is a generic term for unwanted interference energies either conducted as currents or radiated as electromagnetic fields. EMI can emanate from computers or other digital devices in several ways. Generally, voltages and currents in board mounted, integrated switching circuits create electric and magnetic fields that radiate from the device. EMI radiating from such devices will vary in field strength and impedance according to the shape and orientation of the conductors, the distance from the conductors to any shielding provided by circuit components or by coupling to circuit components.
Since most computers operate by generating pulse signals at rates in excess of a million pulses per second, radiating EMI will extend into the radio frequency spectrum and can cause significant interference with radio and television signals. The United States Federal Communications Commission has promulgated rules and procedures requiring the minimization and regulation of EMI by manufacturers. Under such regulations, EMI must be limited for certain devices to particular micro-volt ranges depending on the frequency of the EMI signal.
In lieu of the problems surrounding EMI and the need to comply with government regulations, various schemes have been proposed for limiting EMI. One typical scheme has been to provide a conductive enclosure to an electronic device so that EMI field lines will terminate on such enclosure. Unfortunately, field lines may still emanate from such enclosures, particularly at the point of connection with the printed circuit board, i.e. at the connector.
Consequently, a need exists for controlling EMI emanating from a printed circuit board, even if contained in an enclosure, whereby EMI emanating from the point of connection to the printed circuit board is eliminated.